At entrances and exits of parking lots, individual parking spaces of parking lots, and the like places, vehicle detection sensors which detect vehicles are commonly installed to manage vehicles going in and out of the parking lots or the individual parking spaces. An example of such vehicle detection sensors is disclosed in, for example, Patent Literature 1.
Patent Literature 1 discloses a locking device, which is installed in a parking space, for locking a parked vehicle. The locking device includes vehicle detection sensor which detects the presence or absence of a vehicle in a parking space based on changes of magnetic flux traveling from a transmission coil to a reception coil. Further, based on an output status of the vehicle detection sensor, locking and unlocking operations of the locking device are controlled.
Patent Literature 2 discloses a vehicle detection device which detects a series of movements of a vehicle from an entry into a coin-operated parking lot to parking in a given space and a series of movements of a vehicle from a start from a parked position to an exit of the parking lot, by comparing signal levels of signals outputted by a sensor, the signals having frequencies that vary with a velocity of the vehicle, with a plurality of reference levels.